1. Field of the Invention
The present invention relates to an error correction code encoder and decoder, and more particularly to an encoder and decoder using a so-called product code as an error correction code in a system which transmits a code train via a transmission path such as a magnetic recording/reproducing path which may sometimes generate burst errors.
2. Related Background Art
It is known that a so-called interleave method is used to improve an error correction capability of a system having a transmission path such as a magnetic recording/reproducing path which may sometimes generate burst errors. With this interleave method, data after being subject to error correction code encoding and constituting the same encoded block or same error correction code, is distributedly sent to a transmission line.
Such an interleave method used by a video VTR will be described by way of example.
FIG. 1 is a diagram used for explaining a video signal processed by a digital VTR. In FIG. 1, there are shown horizontal scan lines including the first, second, . . . , 525-th line. In this example, a video signal having 525 horizontal scan lines such as an NTSC signal is used.
In VTR, digital video signals of one frame are divided into a predetermined number (P) of blocks. Each block is subject to error correction code encoding, and written in one track. Thus, P tracks are used for recording video signals of one frame.
FIG. 2 is a diagram illustrating error correction code encoded blocks. As shown in FIG. 2, information data (information words) I.sub.11 to I.sub.ki is encoded, for example by using such as Reed Solomon codes to provide inner codes in the horizontal direction and outer codes in the vertical direction, i.e., provide so-called product codes. In this example, it is assumed that inner and outer codes include three parity words (check bits) IP.sub.11 to Ip.sub.m3, and OP.sub.11 to OP.sub.3k, respectively, and that each code can correct one word error.
The order of recording data (order of data transmission) in a magnetic tape is the same as the direction of inner code encoding, i.e., in the order of I.sub.11, I.sub.12, I.sub.13, . . . , I.sub.1i, IP.sub.11, IP.sub.12, IP.sub.13, I.sub.21, I.sub.22, . . . . Therefore, burst errors caused by dropout, scratches or stains on a magnetic tape during a reproducing operation are consecutive in the inner code encoding direction.
As described above, if each outer or inner code can correct one word, burst errors of maximum one line or i words can be corrected by outer codes, but burst errors more than this cannot be corrected.
In order to improve the error correction capability, data is encoded for each block. The encoded data is exchanged between blocks to record mixed data of a plurality of blocks in one track. For example, in a simple case, data of two blocks is exchanged on the line unit basis. With such an arrangement, even if burst errors more than one line occur, burst errors of a maximum of two lines can be corrected using outer codes because the reproducing process can be performed one line per each block. However, in this case, two tracks for the unit interleave process should not contain other burst errors or random errors. It can be said therefore that the above-described interleave method is very effective for improving the error correction capability without having a large redundancy if occurrence frequency of burst errors is fairly low.
FIG. 3 is a block diagram showing the brief arrangement of a conventional digital VTR which uses the interleave method.
Information data inputted from an input terminal 300 is encoded on a block unit basis by an outer code encoding circuit 301 and an inner code encoding circuit 302. The data is exchanged or interleaved in line units or word units with an interleave circuit 303 by using a memory. The interleave circuit 303 sequentially outputs data of a plurality of blocks to a recording circuit 304 which processes the data for magnetically recording it on a magnetic tape (recording medium) 310 serving as a transmission path.
A signal picked up from the magnetic tape 310 is supplied to a reproducing circuit 305 to reproduce data. The reproduced data is processed by another interleave circuit 306, in a manner opposite to the interleave circuit 303, and sequentially outputted one line after another in units of block such as shown in FIG. 2. The outputted data is subject to error correction processing at an inner code decoding circuit 307 and an outer code decoding circuit 308, using inner and outer codes. The error-corrected digital video signal is outputted from an output terminal 309.
The above-described digital VTR requires address generators, memories, and the like, which are used for the interleave process only, resulting in an increase of hardware.
Furthermore, the order of data before encoding is different from the order of data on a magnetic tape (on a transmission line). Therefore, a desired data order before error correction code encoding cannot be recorded (transmitted) as it is. In a special reproducing mode of a digital VTR, such as a high speed search which reproduces data while transporting a tape at a speed different from that when recording it, a particular data order is used for allowing effective pixel reproduction. This particular data order of video signals changes if error correction code encoding is performed. It becomes therefore necessary to rearrange the data after error correction code encoding, or to determine a data order while considering such change in advance. In the former case, complicated processing is required, and moreover this processing is carried out with parity bits (words) being affixed so that data amount to be processed becomes bulky and the amount of hardware increases. Also in the latter case, complicated processing is required, a time required for data exchange may become longer, and the amount of hardware increases.